What causes this? Did I get the metal too hot?
Leonids post today answers your basic question, ie, the metal wasn't
properly annealed before you soldered the two together.
I'd add one note though. Especially on larger items, and especially
when working with metals that, unlike silver, may not be such
excellent conductors of heat, it's sometimes problematic to assemble
things with perfectly flat sheet, as it can sometimes warp,
particularly if other things are already soldered to it. One means to
control this is to very gently planish the sheet metal before
soldering to it. You work mostly in the center, and with softer
metals, you can even use something soft like a rawhide mallet. The
idea is that by very faintly stretching the center of the sheet, you
are producing a very slight domed shape, rather than fully flat. It
does not need to be much. Even barely perceptable. But with that dome
shape, if the metal now wishes to deform or warp, what tends to
happen is that the dome either increases in height or decreases, but
the sheet basically stays in much the same plane. On cooling,
whatever thermal expansion changed the shape, reverses itself, so you
maintain the shape you started with. Way back "when", doing things
like boxes or similar constructions, I'd want a bottom to the box
that sat flat on a surface, without rocking. Very slightly doming the
metal before assembly meant that you could be sure you'd not end up
with a container who's bottom bulged out and didn't sit right.
Working on your cabs, you can do the reverse, insuring that the
bottom, if it no longer is perfectly flat, at least bulges down, not
up, into the bezel, so the stone still fits all the way down.
And one other note. You did say you're working with sterling (very
soft metal, after all) with decent thickness. So rather than
agonizing over the fact that the bottom warped, why not just fix it?
Put the bezel on a nice flat steel surface, and with perhaps even
just a piece of wood dowel as a punch, or if you need, a flat ended
steel punch (perhaps the back end of a dapping punch, or whatever
else you might have around), "hammer" the center back down. It
shouldn't be much trouble to flatten that back surface enough so you
no longer hve a problem fitting the stone... You can also do this
effectively from the back. Hold a suitable steel rod (or anything
else the right shape), with a nice flat end, facing upwards, if a
vise. Now support the bezel's base from the inside, on that surface,
which we'll now be calling a "stake"... You can now gently planish
the back, working mostly the outer edges. The hammer will drive the
sides down, the stake supports the middle, and you end up with a
flat surface again...